Losartan potassium, i.e. 2-n-butyl-4-chloro-1-[(2′-(tetrazol-5-yl)-1,1′-biphenyl-4-il)methyl]-1H-imidazole-5-methanol potassium salt, is a known angiotensin II antagonist widely used in therapy, for example in the treatment of hypertension, having the following formula (I)

According to one of the various known synthetic routes for the preparation of losartan, one of the key intermediates is 1-bromo-4-(2′-butyl4′-chloro-5-hydroxymethylimidazole-1′H-1′-yl) methylbenzene, having the formula c) reported below. The preparation of this intermediate disclosed in WO 93/10106 comprises the reaction between 2-n-butyl-4-chloro-1-H-indazole-5-carboxaldehyde of formula a) and p-bromo-benzyl bromide of formula b) to give the compound of formula c).

This method is not selective, due to the fact that the imidazole compound of formula a) exists in 2 tautomeric forms which both react with the compound of formula b) leading to the formation of the desired compound of formula c) and of its position isomer of formula d). Since the compound of formula d) has a chemical behaviour similar to the compound of formula c), in the cross-coupling reactions that lead to the formation of losartan potassium, it gives rise to an impurity which can be removed only by means of repeated and troublesome crystallizations, which significantly decrease the yield and increase production costs.
There is therefore the need for a new method for preparing the said benzylimidazole intermediate which overcomes the aforementioned drawbacks.